A fiber optic transmission system typically includes:
a transmitter terminal modulating the optical frequency and/or power of at least one optical carrier wave as a function of the information to be transmitted, PA1 an optical transmission link consisting of at least one monomode fiber section conveying the signal output by the transmitter terminal, and PA1 a receiver terminal receiving the optical signal transmitted via the fiber. PA1 a polarization controller, PA1 a differential delay generator for generating a differential delay between two orthogonal polarization modes, said controller and said generator being inserted between the transmission fiber and the receiver terminal, in that order, and PA1 a control unit for controlling the polarization controller, the system further including a second chromatic dispersion compensator inserted between said transmitter and receiver terminals, said second compensator applying compensation of fixed value which tends to minimize the error rate of signals received by the receiver terminal.
The performance of an optical transmission system, in particular in terms of signal quality and bit rate, is limited in particular by optical properties of the link, which is subject to physical phenomena which degrade the optical signals. Of all the phenomena that have been identified, attenuation of the optical power and chromatic dispersion were initially seen as the most severe, and means have been proposed for at least partly remedying the degradation they cause.
The attenuation in fibers of a given type depends on the signal carrier wavelength. Accordingly, monomode fibers installed during the last ten years, referred to as "standard fibers", have minimum attenuation at a wavelength of around 1.5 .mu.m, which makes it beneficial to choose that value for the carriers.
Also, to increase transmission distances further, it has been possible to compensate attenuation by means of optical amplifiers at the upstream or downstream end of the link or all along the link.
Chromatic dispersion also depends on wavelength. For standard fibers, there is zero chromatic dispersion at 1.3 .mu.m and approximately 1.7 ps/(km.nm) of chromatic dispersion at 1.5 pm. The low attenuation at 1.5 .mu.m has led to the development of new fibers, referred to as "dispersion shifted fibers", for which there is zero chromatic dispersion at that wavelength.
Attempts have also been made to correct the effects of chromatic dispersion at 1.5 pm in existing installed standard fibers, in order to improve their performance.
One solution is to insert into the link at least one dispersion compensating fiber (DCF). To compensate the chromatic dispersion exactly, it is sufficient for the dispersion compensating fiber to have a length and dispersion characteristics such that the cumulative dispersion along the compensating fiber is equal and opposite to that along the transmission link fiber.
A residual cumulative dispersion value DR for the whole of the link, including the compensating fiber(s), can be defined as the algebraic sum of the cumulative dispersions DL and DC of the dispersion compensating fiber(s) and the transmission link fiber. In mathematical terms, this can be expressed by the following equation: EQU DR=DC+DL=.intg.D.sub.1 (z.sub.1).multidot.dz.sub.1 +.intg.D.sub.2 (Z.sub.2).multidot.dZ.sub.2 (1)
where z.sub.1 and z.sub.2 are respectively the abscissae of points along the dispersion compensating fiber and along the associated link, D.sub.1 and D.sub.2 are respectively the chromatic dispersion parameters at the abscissae z.sub.1 and z.sub.2 of the dispersion compensating fiber and the transmission link fiber and the integrals which express the cumulative dispersions DC and DL are respectively calculated along the dispersion compensating fiber and along the associated transmission link fiber, taking the wave propagation direction as the positive direction.
The dispersion parameter D is related to the propagation constant .beta. by the following equation: EQU d.sup.2.beta./d.omega..sup.2 =-(2.pi.c/.omega..sup.2)D
where .omega. is the angular frequency of the wave and c is the speed of light in a vacuum.
The condition for exact compensation of the chromatic dispersion is therefore DR=DC+DL=0.
In reality, the optimum of exact compensation of chromatic dispersion is never achieved because the quality of the compensated signal received also depends on other transmission parameters, in particular the type of modulation of the transmitted signal. This applies in particular if the transmitted signal is "chirped", i.e. if optical frequency modulation accompanies any amplitude modulation.
In fact, such compensation is imposed only if required, i.e. for transmission conditions (fiber type, modulation type, transmission distance and bit rates) which, without compensation, would lead to error rates exceeding a commercially acceptable limit value, typically 10.sup.-15. Moreover, to minimize the cost of the dispersion compensating fiber, a minimum compensation value compatible with the required error rate is normally chosen. Accordingly, on sufficiently short links, no attempt at all is made to compensate chromatic dispersion.
Until now, the forms of compensation referred to above have been treated independently and without regard to another unfavorable phenomenon referred to as "modal polarization dispersion". Under existing optical transmission system operating conditions, this phenomenon has long been regarded as negligible compared with chromatic dispersion. This no longer applies if further attempts are made to increase the length of the link and above all the bit rate.
Fibers are subject to polarization dispersion even in the absence of chromatic dispersion in the usual sense, and even though the carrier wave supplied by a laser diode at the transmitter is totally polarized. One effect of polarization dispersion is that, when it is received after propagating in a fiber, a pulse output by the transmitter terminal is distorted and has a duration greater than its original duration.
This distortion is due to birefringence of the fiber, which depolarizes the optical signal during transmission. To a first approximation, the signal received at the end of the line fiber can be considered as made up of two orthogonal components, one corresponding to a state of polarization for which the propagation speed is a maximum (fastest principal state of polarization) and the other corresponding to a polarization state for which the propagation speed is a minimum (slowest principal state of polarization). In other words, an impulse signal received at the end of the line fiber can be considered to be made up of a first impulse signal with a privileged state of polarization and arriving first and a second impulse signal propagating in a delayed state of propagation and arriving with a differential group delay (DGD) which depends in particular on the length of the line fiber. These two principal states of polarization (PSP) therefore characterize the link.
Consequently, if the transmitter terminal outputs an optical signal consisting of a very brief pulse, the optical signal received by the receiver terminal is made up of two successive and orthogonally polarized pulses having a relative time shift equal to the DGD. As detection by the terminal entails providing in electrical form a measurement of the total optical power received, the detected pulse will have a temporal width increased as a function of the DGD value.
The delay can be in the order of 50 picoseconds for a standard fiber 100 kilometers long. The distortion of the pulses received by the receiver terminal can cause errors in decoding the transmitted data, and polarization dispersion therefore constitutes a factor limiting the performance of optical links, whether analog or digital.
At present monomode fibers can be fabricated with low polarization dispersion (approximately 0.05 ps/km). However, the problem remains in the case of installed "standard" fibers, which have very high polarization dispersion constituting a major technical obstacle to increasing the transmitted bit rates. The problem will also occur on seeking to increase further the bit rate of low polarization dispersion fibers.
Fibers with high polarization dispersion, also referred to as polarization maintaining fibers (PMF), can be used in short lengths to procure a fixed differential delay with invariant principal states of polarization. Polarization dispersion can be compensated optically by judicially placing a component of this kind (or any system for generating a differential delay between two orthogonal polarization modes) in series with a transmission link subject to polarization dispersion. This can be achieved either by using a polarization maintaining fiber with the same differential delay as the link, but interchanging the slow and fast principal states of polarization, or by making a principal state of polarization of the combination of the link and the polarization maintaining fiber coincide with the state of polarization of the source of the transmission. This is done using a polarization controller between the link and the polarization maintaining fiber.
One important aspect of modal polarization dispersion is that the differential delay DGD and the principal states of polarization of a link vary in time as a function of many factors, including vibration and temperature. Accordingly, unlike chromatic dispersion, polarization dispersion must be considered a random phenomenon. In particular, the polarization dispersion of a link is characterized by a polarization mode dispersion delay (PMD) defined as the average value of the measured DGD. To be more precise, it can be shown that the polarization dispersion can be represented by a vector of random rotation O in the Stokes vector space usually employed to represent the states of polarization by means of the Poincare sphere.
Another consequence of this random aspect is that a compensator system must be adaptive, and the differential delay of the polarization maintaining fiber must be made at least equal to the differential delay values to be compensated. A compensator system of the above kind is described in European Patent Application EP-A-853 395 filed Dec. 30, 1997 and published Jul. 15, 1998.
A problem which has come to light in research on PMD compensation is the combined influence of polarization dispersion and chromatic dispersion. It has become apparent that in reality PMD compensation is highly sensitive to the residual chromatic dispersion of the link as a whole, and therefore to the existence and amount of chromatic dispersion compensation.
In particular, it has been found necessary to introduce precise chromatic dispersion compensation even on links where such compensation would not have been necessary in the absence of PMD.
It has also become apparent that the optimum chromatic dispersion compensation to be applied in the presence of PMD does not always correspond to the optimum compensation that would be applied in the absence of PMD.